云南大学中国西南天文研究所研究团组发布国际上首个具有精确距离测量的大样本分子云表

The first large catalogue of molecular clouds in the Galactic plane with accurate distance measurements

近日，云南大学中国西南天文研究所陈丙秋副教授、李广兴副教授和刘晓为教授利用本团组绘制的国际上首张完整覆盖银盘的高精度三维尘埃分布图，搜寻银盘的分子云并精确测量它们的距离，发布了国际上首个具有精确距离测量的银盘大样本分子云表。

银河系中大部分的分子气体存在于低温、稠密的巨大星云，即分子云中。分子云的主要成分是分子氢，也包含极少量的其他分子如一氧化碳分子和星际尘埃。分子云温度只有10 – 30开尔文 （零下260 – 零下250摄氏度），其典型大小为100多光年，典型质量为10万个太阳质量。分子云主要分布在银盘中，银河系内大约存在数千个分子云，它们是恒星形成的场所，理解分子云的物理特征对于研究恒星形成的物理过程乃至银河系的恒星形成历史与演化过程具有重要的意义。

精确测量分子云的距离是获取分子云物理特征的基础，但对分子云距离的测量十分困难。传统的基于分子云视向速度获得的运动学距离具有30-50%的误差，只有极个别太阳附近的著名的分子云才有精确的距离测量，所以建立一个具有精确距离测量的银盘大样本分子云表至关重要。云南大学中国西南天文研究所团组基于本团组绘制的国际上首张完整覆盖银盘的高精度三维尘埃分布图 （见http://www.swifar.ynu.edu.cn/info/1111/1931.htm），采用了一种聚类分层算法，分离出银盘中567个分子云，并基于本团组之前的研究算法，给出了这些分子云精确的距离，精度可达2 - 5%。这是目前国际上首个具有精确距离测量的大样本分子云表。基于这个样本，研究团组还发现了银河系旋臂中一个新的毛刺子结构（spur）。该研究工作已在国际专业学术期刊《皇家天文学月刊》（Monthly Notices of the Royal Astronomical Society）发表（文章链接：https://academic.oup.com/mnras/article/ 493/1/351/5716681）。

图. 样本中分子云的空间分布（红色圆点），背景为NASA/JPL-Caltech/R. Hurt (SSC/Caltech)绘制的银河系正面假想图。Spatial distribution of molecular clouds from Chen et al. (2020).

    Most molecular gas in our Galaxy is contained in molecular clouds, where the star formation occurs。The formation of molecular clouds and the birth of stars are key processes in the life cycle of galaxies. Understanding the physical properties of molecular clouds is thus of great importance. For that, accurate distance measurements for the clouds are essential to estimate their physical quantities including size and mass, etc.

    However, building a large sample of molecular clouds with accurate distance measurements is a difficult task. Kinematic distances estimated from light-of-sight velocities suffer from large uncertainties due to the difficulties of determining an accurate rotation curve and the influence of the peculiar velocities and the noncircular motions, as well as the well-known near-far ambiguity. Most of the molecular clouds are located in the Galactic disk. However, hitherto only a few local well-studied molecular clouds at low latitudes have accurate distance estimates. In our study, we identify and isolate 567 molecular clouds in the Galactic plane by applying a hierarchical clustering analysis technique to the 3D colour excess maps of the Galactic disk previously constructed by us (http://www.swifar.ynu.edu.cn/info/1111/1931.htm). We also obtain their distance estimates by a dust model fitting algorithm. The typical distance uncertainty is 2 - 5 per cent. As far as we know, this is the first large catalogue of molecular clouds in the Galactic plane with distances derived in a direct manner. Based on the catalogue, we propose the existence of a possible spur, with a pitch angle of about 34°, connecting the Local and the Sagittarius Arms in the fourth quadrant. The paper has been published in the Monthly Notices of the Royal Astronomical Society (article link: https://academic.oup.com/mnras/article/ 493/1/351/5716681).